Why is a trailing comma a SyntaxError in an argument list that uses *args syntax?

Question

Why can't you use a trailing comma with *args in Python? In other words, this works

>>> f(1, 2, b=4,)

But this does not

>>> f(*(1, 2), b=4,)
  File "<stdin>", line 1
    f(*(1, 2), b=4,)
                   ^
SyntaxError: invalid syntax

This is the case with both Python 2 and Python 3.

Answer 1

Let's look at the language specification:

call                 ::=  primary "(" [argument_list [","]
                          | expression genexpr_for] ")"
argument_list        ::=  positional_arguments ["," keyword_arguments]
                            ["," "*" expression] ["," keyword_arguments]
                            ["," "**" expression]
                          | keyword_arguments ["," "*" expression]
                            ["," "**" expression]
                          | "*" expression ["," "*" expression] ["," "**" expression]
                          | "**" expression
positional_arguments ::=  expression ("," expression)*
keyword_arguments    ::=  keyword_item ("," keyword_item)*
keyword_item         ::=  identifier "=" expression

Let's sift down to the parts we care about:

call                 ::=  primary "(" [argument_list [","]] ")"
argument_list        ::=  positional_arguments ["," keyword_arguments]
                            ["," "*" expression] ["," keyword_arguments]
                            ["," "**" expression]
positional_arguments ::=  expression ("," expression)*
keyword_arguments    ::=  keyword_item ("," keyword_item)*
keyword_item         ::=  identifier "=" expression

So, it looks like after any arguments to a function call, we're allowed an extra ,. So this looks like a bug in the cpython implementation.

Something like: f(1, *(2,3,4), ) should work according to this grammar, but doesn't in CPython.

---

In an earlier answer, Eric linked to the CPython grammar specification, which includes the CPython implementation of the above grammar. Here it is below:

arglist: (argument ',')* ( argument [',']
                         | '*' test (',' argument)* [',' '**' test] 
                         | '**' test
                         )

Note, that this grammar is not the same as the one proposed by the language specification. I'd consider this an implementation bug.

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Note that there are additional issues with the CPython implementation. This should also be supported: f(*(1,2,3), *(4,5,6))

Oddly though, the specification does not allow f(*(1,2,3), *(4,5,6), *(7,8,9))

As I look at this more, I think this part of the specification needs some fixing. This is allowed: f(x=1, *(2,3)), but this isn't: f(x=1, 2, 3).

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And to perhaps be helpful to the original question, in CPython, you can have a trailing comma if you don't use the *args or the **kwargs feature. I agree that this is lame.

Answer 2

After some discussion regarding this bug in issue 9232, Guido van Rossum commented:

I'm +1 on adding this. I don't believe it requires a PEP. A trailing comma in definitions is already supported in some places, so I don't buy the argument that it catches errors. During the moratorium we were perhaps too strict.

Subsequently, a patch by Mark Dickinson was committed. So this is now fixed in Python 3.6.0 alpha 1.